Consider a non-rigid rotating system where the rotational velocity is given by the Kepler speed. In such system, adjacent orbits have a different orbital speed of $$ v \approx \sqrt{\frac{GM}{r}}.$$
Thus the orbital velocity drops the further you go outward.
Place yourself onto one particle and you see that the elements outward are slower than you, while the elements further inward are faster. A velocity shear is $dv / dr$. Adding "Keplerian" tells you (1) how fast the velocity is changing, and (2) that the shear is in the radial direction. A shear $dv / dz$ would be in a different direction, so it would not make sense to call it Keplerian because the Keplerian velocity depends only on radial distance.
In a protoplanetary disk, such shear motion counter-acts self-gravity of the small particles in a way it has mass further inside rotate quicker. With sufficiently dense disk, thus solid mass, and sufficient gas in the disc, this may actually lead to hydrodynamic instabilities forming vortices which locally counter this shear - and essentially only then allow planetesimals to form. In order to assess stability in such rotating disc with shear, one uses the Toomre criterion (which is basically the Jeans stability criterion expanded to a rotating disc).
Similarily Keplerian shear can be observed in the Saturnian rings where in the absence of gas and hydrodynamics it basically stops the growth of the so-called moonlets.
